I do not know what "to fail at one's peril" means. The phrase appears in legal documents until the 19th century (at least this is what Google suggests). I cannot deduce its meaning no matter how hard I try.
closed as general reference by FumbleFingers, Gnawme, kiamlaluno, Matt Эллен, Mitch Feb 10 '12 at 2:53
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The complete sentence is often something like, "Whereof you may not fail, at your peril." This means that if you fail, it will be perilous for you.